Make llama stack build not create a new conda by default (#788)

## What does this PR do?

So far `llama stack build` has always created a separate conda
environment for packaging the dependencies of a distribution. The main
reason to do so is isolation -- distributions are composed of providers
which can have a variety of potentially conflicting dependencies. That
said, this has created significant annoyance for new users since it is
not at all transparent. The fact that `llama stack run` is actually
running the code in some other conda is very surprising.

This PR tries to make things better. 

- Both `llama stack build` and `llama stack run` now accept an
`--image-name` argument which represents the (conda, docker, virtualenv)
image you want to operate upon.
- For the default (conda) mode, the script checks if a current conda
environment exists. If one exists, it uses it.
- If `--image-name` is provided, that option is used. In this case, an
environment is created if needed.
- There is no automatic `llamastack-` prefixing of the environment names
done anymore.


## Test Plan

Start in a conda environment, run `llama stack build --template
fireworks`; verify that it successfully built into the current
environment and stored the build file at
`$CONDA_PREFIX/llamastack-build.yaml`. Run `llama stack run fireworks`
which started correctly in the current environment.

Ran the same build command outside of conda. It failed asking for
`--image-name`. Ran it with `llama stack build --template fireworks
--image-name foo`. This successfully created a conda environment called
`foo` and installed deps. Ran `llama stack run fireworks` outside conda
which failed. Activated a different conda, ran again, it failed saying
it did not find the `llamastack-build.yaml` file. Then used
`--image-name foo` option and it ran successfully.

llama_stack/providers/inline/inference/meta_reference/quantization/scripts/build_conda.sh

@@ -1,36 +0,0 @@
-#!/bin/bash
-
-# Copyright (c) Meta Platforms, Inc. and affiliates.
-# All rights reserved.
-#
-# This source code is licensed under the terms described in the LICENSE file in
-# the root directory of this source tree.
-
-if [[ $# -ne 1 ]]; then
-    echo "Error: Please provide the name of CONDA environment you wish to create"
-    exit 1
-fi
-
-ENV_NAME=$1
-
-set -eu
-eval "$(conda shell.bash hook)"
-
-echo "Will build env (or overwrite) named '$ENV_NAME'"
-
-set -x
-
-run_build() {
-    # Set up the conda environment
-    yes | conda remove --name $ENV_NAME --all
-    yes | conda create -n $ENV_NAME python=3.10
-    conda activate $ENV_NAME
-
-    # PT nightly
-    pip install --pre torch --index-url https://download.pytorch.org/whl/nightly/cu121
-
-    # install dependencies for `llama-agentic-system`
-    pip install -r fp8_requirements.txt
-}
-
-run_build